The brushless motor typically includes a permanent magnet rotor and a stator made up of a number of windings that may be connected in a star configuration or completely independent from each other. In the majority of cases, brushless motors have three-phase windings. In addition, the driving circuit may be integrated and have an output stage that drives the phase windings using a three-phase full-wave bridge circuit employing six MOS or bipolar power transistors.
FIG. 1 represents a driver stage and the electrical diagram of a DC brushless motor connected to the driver stage. A most typical mode of driving this type of motors is the so-called bipolar mode where at any instant two phase windings are powered and the third remains idle, that is, the output of the respective full-wave bridge is kept in a state of high impedance (Hi Z).
The phase windings are switchingly driven according to a cyclic sequence which must be synchronized with the rotor's instantaneous position. In a bipolar driving mode, the instantaneous position may be determined by monitoring the induced back-electromotive-force (BEMF) on the unpowered phase winding or detected by sensors (a more expensive approach that is seldom used).
Another driving mode for this type of motor is the so-called tripolar mode, or, more precisely and generally, the mode according to which all the motor's phase windings are constantly powered. This is a technique wherein detecting of the BEMF on a momentarily unpowered winding cannot be exploited and more sophisticated techniques must be resorted to.
To optimize the system's performance, the power supplied to the phase windings should be such as to operate the motor at its maximum efficiency. This is attained by driving the phase windings with a precise phase between the current forced through the phase winding and the relative BEMF induced thereon.
The torque in a permanent magnet synchronous motor, such as the brushless DC motor, is produced by the stator's current component that creates a magnetic field in quadrature with the rotor's magnetic field. By calling i.sub.d and i.sub.q the stator's current components that respectively generate a magnetic field in phase and in quadrature with the rotor's magnetic field, the maximum efficiency is obtained for i.sub.d =0. To ensure that the whole stator's current generates a field in quadrature it is necessary to force the current through each phase winding in phase with the relative BEMF.
Driving the motor in a bipolar or even in a unipolar mode implies a certain ripple in the produced torque characteristic. The bipolar or unipolar driving does permit an easy detection of the BEMF on the winding or on the windings that are not excited to derive the required synchronous information to effect the phase switchings in perfect phase with the rotor position.
As a matter of fact, systems capable of implementing a tripolar driving mode have been developed to reduce or eliminate the torque ripple. In a tripolar driving mode all the phase windings are constantly driven by alternating (sinusoidal) signals, mutually out of phase from one another. Hence, to be able to "read" the BEMF signal, a momentary interruption of the driving of at least one phase winding of the motor, is effected by placing the respective driving bridge in a condition of high impedance (tristate). This is done for a time interval sufficient to detect a zero-cross event of the BEMF signal. From this information a synchronization signal of the motor's phase switchings is reconstructed. Nevertheless, these techniques introduce a certain torque ripple as compared to a pure tripolar driving mode, that is, without any interruptions for detecting the BEMF signal.
Many driving systems are designed to switch from a unipolar and/or bipolar mode to a tripolar mode and vice-versa. In practice, the motor is started and brought to a steady state by driving it in a unipolar and/or bipolar mode and, once the steady state speed is reached, the system switches to a tripolar driving mode to reduce or eliminate the torque ripple of the motor operating in a steady state condition. If Hall effect sensors are not employed, an alternative technique is based on the use of relatively complex electronic circuits capable of reconstructing a BEMF signal once the motor's electrical parameters (resistance and inductance) are known. The reconstructed BEMF signals are resorted to so as maintain a correct synchronization of the phase switchings during the operation in a tripolar mode.
It is evident that there is a need or usefulness of a method and a system capable of driving the motor at a steady state speed in a pure tripolar mode, thus minimizing or nullifying the torque ripple. This would also be desirable without driving interruptions necessary to monitor the BEMF or implementing complex reconstructing systems of the BEMF signal while ensuring a perfect synchronization of the phase switchings with the rotor's instantaneous position and optimizing the efficiency.